In a Radio-Frequency Integrated Circuit (IC) for inputting and outputting a high frequency signal, a variable gain amplifier for changing a gain is disposed in order to obtain a high frequency signal of electric power in a desired range. When the amplifier is provided with a gain variable function, a performance (for example frequency characteristics, noise figure characteristics, and gain linearity) of the amplifier itself may be degraded.
For example, in a receiver, a low noise amplifier (LNA) for amplifying a signal received by a receiving antenna is usually disposed in a following stage of the receiving antenna. For example, when the low noise amplifier has the variable gain function, performance degradation of the low noise amplifier remarkably appears, resulting in performance degradation of the receiver. For example, when a millimeter wave signal of several tens of GHz is handled, an influence of performance degradation of a transistor as an amplifying element used in the low noise amplifier is significant.
As related art with respect to the low noise amplifier having the variable gain function, Non Patent Literature 1 is known. A low noise amplifier disclosed in Non Patent Literature 1 includes a plurality of transistors connected in multiple stages, and a DC gate voltage value applied to a gate of each transistor is variable, and thus it is possible to adjust a current value flowing through the transistor. Accordingly, the low noise amplifier can change a gain.
FIG. 14 is a graph illustrating a relationship between the DC gate voltage applied to the gate of the transistor and the current flowing through the transistor. When a DC gate voltage which is higher than or equal to a threshold voltage V_th of the transistor is applied to the transistor, the current flowing through the transistor increases according to an increase in the DC gate voltage. A gradient of the current with respect to the DC gate voltage gradually increases after the DC gate voltage exceeds the threshold voltage V_th, and thus has linear characteristics.
In order to increase gain linearity of the low noise amplifier, it is preferable to use the DC gate voltage (for example, a DC gate voltage V_h) in which the gradient of the current with respect to the DC gate voltage is in a linear region. However, in the low noise amplifier for changing the gain by changing the current flowing through the transistor according to the DC gate voltage (for example, refer to Non Patent Literature 1), for example, when the gain is changed into a low gain, gain linearity of the low noise amplifier may be degraded by using the DC gate voltage (for example, a DC gate voltage V_1) in which the gradient of the current with respect to the DC gate voltage is in a nonlinear region.
In addition, since the threshold voltage V_th varies according to process variations in a manufacturing process of the IC, when a low DC gate voltage (for example, the gate voltage V_1) is used, the gradient of the current with respect to the DC gate voltage is likely to be nonlinear compared to a case where a high DC gate voltage (for example, the DC gate voltage V_h) is used. For this reason, the gain characteristics of the low noise amplifier when the threshold voltage V_th is varied may be further degraded than the gain characteristics of the case where the high DC gate voltage (for example, the DC gate voltage V_h) is used.
In order to solve the above problem, a method for changing the gain by changing a circuit constant of an input matching circuit or an output matching circuit disposed in a previous stage or a following stage of the transistor without changing the DC gate voltage of the transistor is proposed (for example, refer to Patent Literatures 1 to 3).
A double-tuned transformer disclosed in Patent Literature 1 includes a grounded capacitor connected to a first tuning winding wire in an antenna tuning transformer disposed in the following stage of the receiving antenna. The double-tuned transformer changes a capacitance value of the grounded capacitor, and thus input impedance is matched, and the gain is changed.
An antenna matching circuit disclosed in Patent Literature 2 is disposed between an antenna and a tuner module, and includes an inductor, a capacitor, and a PIN diode. The antenna matching circuit turns the PIN diode ON or OFF according to a signal output by the tuner module connected to a following stage. The antenna matching circuit changes the impedance according to ON of the PIN diode, and thus the antenna matching circuit is changed from an antenna matching state to an antenna non-matching state, and the gain is changed.
In a variable load circuit connected to an output stage of a conductance circuit, a gain variable amplifying circuit disclosed in Patent Literature 3 adjusts a capacitance value of a variable capacity, and thus the gain is changed.